We present an extension of the model of
[CITE] that allows us to reconstruct the time
evolution of both the total and the open magnetic flux at
the solar surface since 1700. The flux emerging in large active
regions is determined using the sunspot number as a proxy, while
the flux emergence in small ephemeral regions is described by an
extended cycle whose amplitude and length are related to the
corresponding sunspot cycle. Both types of regions contribute to
the open flux, which is the source of the heliospheric field. The
overlap of the activity cycles of ephemeral regions leads to a
secular variation of the total cycle-related magnetic flux
(active region flux + ephemeral region flux + open flux). The
model results indicate that the total surface flux has doubled in
the first half of the last century. The evolution of the open
flux is in good agreement with the reconstruction by
[CITE].

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